A water plant operator must verify that the free residual chlorine at the consumer end meets standards, while separately documenting combined chlorine. Which test allows free and combined chlorine to be distinguished from each other?
- A Orthotolidine test read after 30 minutes
- B DPD test ✓
- C Phenolphthalein alkalinity test
- D Membrane filtration technique
Correct answer: B. DPD test
Explanation
The DPD (diethyl-p-phenylene diamine) test produces an immediate pink-red colour with free chlorine, and combined chlorine reacts only after further steps such as addition of potassium iodide, so the two fractions can be measured separately. Orthotolidine reacts with both free and combined chlorine together, giving a single yellow colour, which is precisely its limitation. Alkalinity testing and membrane filtration do not measure chlorine fractions.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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